#!/usr/bin/perl --
use strict;
use warnings;
Main( @ARGV );
exit( 0 );
sub Main {
    Mi6();
    print '#'x 33, "\n";
    Mip();
}

sub Mi6 {
    my $list  = [ 1, 5, 2, 3, 4 ];
    DD( $list );
    DD( [ p6quicksort(@$list) ] );
    $list  = [ 3,2,4,5,1 ];
    DD( $list );
    DD( [ p6quicksort(@$list) ] );
}

sub Mip {
    my $list  = [ 1, 5, 2, 3, 4 ];
    DD( $list );
    DD( [ IPquicksort(@$list) ] );
    $list  = [ 3,2,4,5,1 ];
    DD( $list );
    DD( [ IPquicksort(@$list) ] );
}

sub IPquicksort {
    my( @array ) = @_;
    IPquicksortREF( \@array, 0, $#array );
    return @array;
}

sub IPpartition {
    my ( $array, $left, $right, $pivotIndex ) = @_;
    my $pivotValue = $array->[$pivotIndex];
#~ Move pivot to end
#~     swap( $array->[$pivotIndex], $array->[$right] );
    @{$array}[$pivotIndex, $right]  = @{$array}[ $right, $pivotIndex];
    my $storeIndex = $left;
    for my $i ( $left .. ( $right - 1 ) ) {             # left = i < right
        if ( $array->[$i] < $pivotValue ) {
#~             swap( $array->[$i], $array->[$storeIndex] );
            @{$array}[ $i, $storeIndex ] = @{$array}[$storeIndex , $i];
            $storeIndex = $storeIndex + 1;
        }
    }

#~ Move pivot to its final place
#~     swap( $array->[$storeIndex], $array->[$right] );
    @{$array}[$storeIndex, $right] = @{$array}[$right, $storeIndex ];
    return $storeIndex;
}

sub IPquicksortREF { 
    my ($array, $left, $right) = ( @_, 0, 0 );
    $right ||= $#$array;
#~ If the list has 2 or more items
    if( $left < $right ){
#~ See "Choice of pivot" section below for possible choices
#~ choose any $pivotIndex such that $left <= $pivotIndex <= $right
        my $pivotIndex = $left;
#~ Get lists of bigger and smaller items and final position of pivot
        my $pivotNewIndex = IPpartition($array, $left, $right, $pivotIndex);
 
#~ Recursively sort elements smaller than the pivot
        IPquicksortREF($array, $left, $pivotNewIndex - 1);
 
#~ Recursively sort elements at least as big as the pivot
        IPquicksortREF($array, $pivotNewIndex + 1, $right );
    }
}
    
sub p6quicksort {
    return @_ if @_ < 2;
    my( $pivot , @rest ) = @_;
    # Partition.
    my @before = grep { $_ < $pivot } @rest;
    my @after  = grep { $_ >= $pivot } @rest;
    # Sort the partitions.
    return ( p6quicksort(@before), $pivot, p6quicksort(@after));
}
sub DD {
    use Data::Dumper();
    print STDERR Data::Dumper->new([@_])->Indent(0)->Dump, "\n";
}

__END__
$VAR1 = [1,5,2,3,4];
$VAR1 = [1,2,3,4,5];
$VAR1 = [3,2,4,5,1];
$VAR1 = [1,2,3,4,5];
#################################
$VAR1 = [1,5,2,3,4];
$VAR1 = [1,2,3,4,5];
$VAR1 = [3,2,4,5,1];
$VAR1 = [1,2,3,4,5];